1. Field of the Invention
The present invention relates to surveillance systems and methods in general, but more specifically, it relates to an improved surveillance system and method using gated-oscillations for the transmitted signal(s), and microprocessor processing of the returned signal(s).
2. Description of the Prior Art
Except in a few elaborate installations, active ultrasonic intrusion detectors are usually continuously transmitting devices. When a continuous signal is transmitted, the returned or received signal(s) is a complex sum of the many reflections of the transmitted signal(s). Under steady-state conditions, the returned signal is a composite that is a sinusoidally variant when the transmitted signal(s) is sinusoidal. The maximum amplitude of the composite will depend upon the location of the receiver, the frequency of the transmitted signal(s), the position of the room furnishings, the location of personnel within the room as well as the amplitude, i.e., power of the transmitted signal(s). If there is a strong background noise situation, due to air currents or other sources, the continuously transmitting type of active ultrasonic detector apparatus, whether it is of the composite maximum amplitude or doppler shift variety, will experience operational difficulties that are manifested in the form of a substantial number of false alarms.
In order to overcome problems like the foregoing, some prior art systems have used pulse-like transmitted signals alone or in combination with doppler shift techniques. In a doppler shift system, the frequency of the reflected signal(s) increases when an object or person approaches the detector, and the frequency of the reflected signal(s) decreases when an object or person moves away. In certain types of situations, the foregoing general type systems operate with a high degree of reliability and in other situations they do not.
The prior art, as indicated hereinabove, includes advances in ultrasonic intrusion detector apparatuses and methods. However, insofar as can be determined, no prior art intrusion detector apparatus or method incorporates all of the features and advantages of the present invention.